Thermal ablation involves the creation of temperature changes sufficient to produce coagulation necrosis in a specific volume of tissue within a patient, typically one or more benign and/or cancerous tumors. In the case of the application of temperatures elevated to above about 50 degrees C., the proper application of heat can result in tissue destruction primarily due to the destruction of proteins within the cells. In the case of reducing the temperature of the targeted area, cycles of proper freezing and thawing can result in tissue destruction primarily due to cell rupture.
Traditional methods of treating cancerous tumors include surgery to physically remove the tumor, chemotherapy to provide systemic treatment by chemical means or radiation, which produces apoptosis in the cells treated with radiation. Frequently these methods are combined to produce the greatest chance of cure. Although these procedures may be life saving, there are serious side effects and risks associated with radiation, chemotherapy, and surgery, any of which may significantly affect patient quality of life.
As a result, there is increasing interest and development of non-invasive or minimally invasive methods to kill tumor cells. In particular, thermal ablation is being investigated as an alternative and/or supplement to traditional methods of tumor destruction. Several methods have been developed and are being developed for various forms of cancer including, among others, cancers of the breast, prostate, lung, kidney, and liver. Methods of introducing localized heat include Radio Frequency Ablation (RFA), microwave therapy, extracorporeal or direct focused ultrasound, laser ablation, and other interstitial heat delivery methods including therapeutic ultrasound applicators. These methods may be applied percutaneously or extracorporeally. Cryoablation, i.e. the freezing of tissue to produce necrosis, is also being used to treat tumors. A significant challenge in ablation therapy is to provide adequate treatment to the targeted tissue while sparing the surrounding structures from injury. Furthermore, current methods of follow-up assessment of the effectiveness of an ablation procedure involve reviewing follow-up images independent from planning and/or ablation procedure monitoring data.
RFA uses electrical energy transmitted into a Volume of Interest (VOI) through an electrode to generate heat in the area of the electrode tip. The radio waves emanate from the non-insulated distal portion of the electrode. The introduced radiofrequency energy causes ionic agitation in the area surrounding the electrode as the current flows from the electrode tip to ground. The resulting agitation causes the temperature in the area surrounding the electrode tip to rise. Temperature calibration or measurement devices, for example thermocouples, in the electrode may provide feedback and allow precise control of the temperatures produced at the electrode tip, while other devices rely on tissue impedance changes to indicate tissue thermal injury. In microwave therapy, applicators function as antennae that concentrate the transmitted microwave energy around the antennae. As in microwave ovens, polar molecules attempt to align themselves with the shifting electromagnetic fields resulting in movement, friction and subsequent heating of the area around the antennas.
Extracorporeal or direct focused ultrasound ablation uses focused sound waves to deliver enough energy to heat a specific volume of tissue to cause coagulation necrosis. To produce coagulation necrosis in larger volumes of tissue the target point is rastered across the target area. Prior to being focused, the sound waves pass through tissue without causing significant heating, only causing destructive heat around the focal point. Therefore, extracorporeal focused ultrasound ablation may be performed without an incision. Laser ablation uses high intensity light to raise the temperature of a target area to produce coagulation necrosis in that area. Generally, needles or applicators containing thin optical fibers are interstitially placed within a tumor. The intense light is transmitted through the optical fibers to the applicator tip and scattered into the targeted area.
Post procedure assessment may also occur. Assessment of tumor response to treatment is used both for the treatment of an individual and for use in clinical trials. A standard for objective assessment currently being used is the RECIST (Response Evaluation Criteria in Solid Tumors) standard. In a typical implementation, the RECIST standard involves measuring a maximum linear dimension on a two-dimensional slice of an image of a tumor (or multiple tumors) and comparing the measurement to previous and subsequently similarly obtained images to estimate tumor response.
Various methods of thermal ablation are being investigated for various types of cancer and various tumor types. For example, cryoablation, focused ultrasound ablation, RFA, microwave thermal ablation, and interstitial self-regulating thermal rods, have all been the subject of studies of the treatment of prostate cancer. However, significant challenges remain with respect to an approach for planning and performing thermal ablation.